Note: Great set of tutorials, that you should definitely go through . Most of them are using MIT App Inventor, which is a drag and drop builder...I mention that resource in the Other Sources category, later in the article...
if you want the slightly more complex one, which you see all the code: https://www.youtube.com/playlist?list=PLGLfVvz_LVvQUjiCc8lUT9aO0GsWA4uNe